46 ANNUAL REGISTER, 1797. 
there is no communication of heat 
between one particle of air and an- 
other particle of air. And from 
hence ‘it follows, that though air 
may, and certainly docs, carry off 
heat, and ‘tranfport it from one 
piace, or from one body to another, 
yet a mafs of air ina quiefcent flate, 
or with all its particles at reft, could 
it remain in this ftate, would be to- 
tally impervious to heat; or fuch a 
ma{s of air would be a perfeét non- 
conductor. ee 
Now if: heat pafles in a mafs of 
air merely in ‘confequence of the 
motion it occafions in that air, if it 
is tranfported, not fuffered to pats, 
—in that cafe it is'clear that what- 
ever can obftruct and impede’ the 
internal motion of the air, muft tend 
to diminith its conducting power: 
and this I have found to be the cafe 
in fact. I found that a certain quan- 
tity of heat which was able to make 
/its way through a wall, or rather a 
feet of confined air, half an inch 
thick in nine minutes and three- 
fifths, required twenty-one minutes 
and two-fifths to make its way 
through the fame wall, when the 
intérnal motion of this air was im- 
peded by mixing with it one-fifty- 
ixth part of its bulk of eider- 
down, of very fine fur, or of fine 
filk, as fpun by the worm. 
But in mixing bedies with air, in 
order to impede its internal motion, 
and render it more fit for confining 
heat, fuch bedies only mut be cho- 
fen as are themfelves non-conduc- 
tors of heat, otherwife they will do 
rnore harm than good, as I have 
found by experience. When, in- 
flead of making ufe of eider-down, 
fur, or fine filk, for impeding the 
internal motion’ of the confined air, 
Tufed an equal volume of exceed- 
isgly fine filver-avire flatted (bving™ 
the ravellings of gold or filver lace} 
the paffage of the heat through the 
barrier, fo far from being impeded, 
was remarkably facilitated by this 
addition; the heat pafhng through 
this compound of air and fine threads 
of metal much fooner thanit would 
have made its way through the air 
alone.” p vB) 
Another circumftance to be at- 
tended to in the choice of a fub- 
ftance to be mixed with air, in or- 
der to form a covering or barrier for 
confining heat, is the finenefs or 
fubtility of its parts; for the finer 
they are, the greater will be their 
furface in proportion to their folidi- 
ty, and the more will they impede 
the motion of the particles of the 
air. Coarfe horfe-hair would be 
found to anfwer much. worfe for 
this purpofe than the fine fur of a 
beaver, though it is not probable 
that there is any eflential difference 
in the chymical properties of thofe 
two kinds of hair. i 
‘ But it is not only the finenefs of 
the parts of a fubftance, and its be- 
ing a non-conduétor, which rendér 
it proper to be employed in the for- 
mation of covering to confine heat ; 
there is ftill another property, more 
occult, which feems to have great 
influence in’ rendering fome fub- 
ftances better fitted for this ufe than 
others; and this is a certain attrac- 
tion which fubfifts between cértain 
bodies and air. The obftinacy with 
which air adheres to the fine fur of 
beafts and to the feathers of birds, 
is well known; and it.may eafily 
be proved that this attraction muft 
affift very powerfully in preventing 
the motion of the air concealed in 
the interftices of thofe fubftances, 
and confequently in impeding the 
paflage of heat through them. 
Perhaps there may be By 
ill 
